Starting at the end of the nineteenth century, crime fighters began to use and develop what has grown into a substantial body of technological tools designed to detect and/or enhance physical evidence. One of the earliest techniques of this kind to receive widespread application is the dusting of fingerprints. Light sources were also among the first tools used in this field. Hence the classic icon of the gumshoe, flashlight in hand, searching for evidence at the dimly lit crime scene.
When a fingerprint is fresh, the oil which forms the print generally follows the pattern of the fingerprint ridges in the finger which made the print. If a fine dust is applied to the surface of a fresh print, the dust tends to adhere to the oils in the fingerprint, thus forming a pattern which generally reveals the pattern of the fingerprint.
Fingerprint dusts were initially selected for their color contrasting qualities as compared to the background. Thus white dust was used to enhance a fingerprint on a black object and vice versa. Even where the oils of a fingerprint have lost their tackiness due to aging or other phenomena, the amino acids into which they break down do cause a minute etching of many surfaces. While this etching is often not visible to the naked eye, and may not become visible with the application of a colored powder, extremely fine fluorescent dusting powders will reveal the fingerprint pattern when illuminated under high intensity light.
Today, many materials, such as dyes, in addition to fluorescent dusting powders are used. Inspection of the evidence is done with specialized light sources. These light sources usually comprise a high intensity source and a filter which passes light having a limited range of wavelengths. Depending upon the material used, which material may be either a fluorescent dusting powder, dye, or other marker material, light having a wavelength which substantially coincides with a known excitation wavelength of the marker is employed. The characteristic of the marker is that, upon illumination with light at one of its excitation wavelengths, it will fluoresce, or emit light. Such fluorescence is typically at a longer wavelength as compared to the excitation wavelength.
Examination of evidence is also enhanced through the use of color filtering glasses or barrier filters, whose color filtering characteristics are tuned to maximize the image to be detected. As noted above, the excitation wavelength is varied through the use of filters at the source. While such devices are very efficient in filtering light, every filter has its own fixed characteristics. These include its center wavelength, bandwidth and transmission coefficient. Thus, if one wishes to have flexibility, it is necessary to have a wide range of filters having different center wavelengths and different bandwidths. This is both cumbersome and expensive. Moreover, as new dyes and powders are introduced, old filters can become obsolete or unnecessary.
In an attempt to provide convenience and flexibility, some light sources used for forensic examination come with a mechanical filter assembly, which allows the introduction of one of about a half dozen filters into the path of the light source to provide the desired wavelength illumination. While this does solve the problem of providing a convenient and easy way to use a light source, obsolescence and limited wavelength and bandwidth selection remain.
In an attempt to overcome some of these disadvantages, earlier forensic illumination systems have attempted to achieve a measure of tunability by mounting an interference filter for angular rotation. Generally, such angular rotation results in a change in angle of incidence with respect to the filter input and a relatively small variation in the encountered path length between the functional layers in the interference filter for light passing through the filter in a fixed direction. In accordance with Bragg's Law, this results in different wavelengths being passed by the filter.
In the above-referenced disclosure of Purcell, a system is disclosed which provided a high intensity light source which is continuously adjustable to vary the center frequency of a band of wavelengths. At the same time, the flexibility of varying the bandwidth of this band was also possible. The same was done with a single light source and a single filtering apparatus. At the same time that was achieved with a mechanical configuration that is both reliable and rugged. Finally, that system was easily portable, and capable of outputting light sufficient for close up analysis of surfaces bearing such material as oils, semen, blood and so forth.
In that system, a method and apparatus for illuminating a deposition of organic material such as, blood, sweat or oil for forensic examination was also provided. A light source emitted light having a range of wavelengths. A first optical coupler or light pipe was positioned and configured to reflect the light toward a reflective diffraction grating. A supportable structure supported, at a selectable relative position, an exit slit and the grating to pass a desired band of wavelengths of output light from portions of the light reflected by the grating. A bendable second optical coupler was coupled to the exit slit and directed the output light toward the deposition to be examined. The bendable second optical coupler comprised a liquid fiber optic member. The support structure rotated the grating. An electronic control and a hand held remote control pad was coupled to the support structure and controlled the support structure.
As can be seen from the above, numerous advantages are provided in such a continuously adjustable diffraction grating based system. Naturally, it is desirable to have the possibility of the highest possible intensity output light at the selected wavelength. However, such a brute force approach results in increased power consumption and excessive heat energy, stressing the rest of the system. In an attempt to achieve better results without aggravating this problem, the above disclosure of Purcell utilizes an infrared blocking filter to filter the light source thus allowing only filtered and relatively low intensity light to fall on the grating. This, however, also has an adverse impact on the amount of energy output by the forensic light source, particularly in the ultraviolet range. In addition, the use of the filters, because they are exposed to a high intensity source, results in there being another element subject to deterioration and replacement.
In accordance with the latter of the two above applications, a filter is used to achieve maximum throughput of energy. Improved signal-to-noise ratio is achieved using other filters at the detection end. In order to protect the filters at the light output side, protection is provided by infrared reflecting filters which reflect unwanted infrared radiation away from the bandpass filters used in the system. However, they also attenuate the desired output light. These bandpass filters are located in the housing and relatively proximate to the source of light which is filtered to output the desired filtered light at the selected wavelength.